The increase in use of satellites in communication and navigation systems requires small antennas for vehicular (car, boat or aircraft) applications. These small antennas must be able to receive circularly polarized radiation even from low elevation angles.
An antenna element in common use today is the microstrip patch antenna which inherently has a very limited frequency bandwidth. This antenna has numerous advantages such as simple fabrication, conformal planar structure, and the existence of many well proven design methodologies and tools. Satellite communications antennas have been built using microstrip patch antennas having metallic radiating elements and producing circularly polarized radiation. In U.S. Pat. No. 4,843,400 a microstrip patch antenna is disclosed which produces circularly polarized radiation using a single feed. The antenna is based on a symmetrical patch with differing dimensions along the axes; however, as many of the existing methodologies and tools have been designed for microwave bands, use of millimeter wave bands requires new antenna design methodologies.
At higher frequencies, metal radiating elements, such as those present in microstrip patch antennas, develop large ohmic losses in conducting surfaces and their effects become significant, also dielectric substrate materials become increasingly dispersive. Designs can not simply be scaled from lower frequencies to higher frequencies without accounting for these factors. Other traditional approaches include the use of multiple monopoles with a reflector and helical antennas both of which have been found to lack robustness and to be difficult to fabricate.
Unshielded dielectric resonators are known to radiate strongly at and around some of their resonant frequencies. Dielectric resonators possess inherent advantages such as high radiation efficiency due to no conductor loss, small size and mechanical simplicity. The radiation pattern, resonant frequency and the operating frequency bandwidth of a dielectric resonator antenna depend on the excited resonant mode, permittivity, the resonator geometry and its surroundings. These provide many degrees of design freedom which may be exploited in controlling antenna characteristics.
Rectangular dielectric resonator antennas have been excited in "magnetic dipole" mode and shown to produce a linearly polarized electric field. To achieve this, a rectangular dielectric resonator antenna is placed on a metallic plane over a small aperture which is excited by a microstripline on the other side of a dielectric substrate. This can also be done using a single probe or monopole antenna placed near the centre of one side of the resonator. The rectangular resonator, and its image in the ground plane combine to form an isolated horizontal magnetic dipole.
If a single element is to be implemented in arrays, the simpler the single-element feed, the simpler the array feed. The limiting case would be a single-feed antenna. It is desirable to minimize the complexity of an antenna feed network so that losses and physical size are lessened. Producing circularly polarized radiation requires two fields mutually orthogonal in both space and time having equal amplitude. Thus, to modify an inherently linearly polarized antenna element (such as the dielectric resonator) such that it is circularly polarized, requires the excitation of two mutually orthogonal modes within the antenna element. This can easily be done with dual feed points, or with an array of properly designed linearly polarized antenna elements. It has now been found that the generation of circularly polarized radiation using a single feed and a single dielectric resonator can be accomplished.